1. Field of the Invention
The present invention relates to an indexable cutting insert for cutting metal machining.
2. Description of Related Art
The vast majority of the variety of different indexable cutting inserts, both single-sided and double-sided, existing today on the market, have a central through hole. This hole is intended for the accomodation of a suitable locking screw for clamping the indexable cutting insert or the cutting insert in a holder. If the cutting insert is intended for turning, the holder is normally a boring bar; if the cutting insert is intended for milling, the holder is for instance a milling cutter body, a long edge cutter or an end mill. Further, the insert may be intended for drilling and may then be clamped by a screw in a drill shaft. As known, the fact that the cutting insert is an indexable one means that one and the same cutting insert has two or more cutting edges, a new cutting edge being positioned or indexed into an operative position when the previous one has been worn out, and this is done by loosening the screw and turning the insert a certain fraction of a revolution.
Indexable cutting inserts are most commonly made of coated or uncoated cemented carbide, but inserts of different ceramic materials also exist. Usually, they have a square, triangular, rhombic, round, rectangular or hexagonal basic shape, but other basic shapes also occur. However, one thing they all have in common, viz. they are normally provided with a through central hole in the geometrical middle point. These holes may have different hole configurations, but a common feature for the vast majority is that the hole is rotation-symmetrical. This implies that the operator has to turn the indexable cutting insert around the loosened screw to substantially the right insert position before he/she tightens the screw again. Normally, the indexable cutting insert is "self-guiding" towards the support surfaces of the insert seat, but if these are small and/or if the insert has a plurality of sides (for instance six or eight), it may happen that the insert is forced into an erroneous intermediate position, and the insert will not be fastened steadily in the intended way and may be broken and/or the insert seat will be damaged.
SE-B-346 074 (& U.S. Pat. No. 3 341 921) discloses a triangular cutting insert with a central hole that also has a triangular basic shape, and also a conicity that widens upwards. The basic shape of the hole intends to offer a double contact surface instead of a single one for the screw in the insert hole, in order to avoid that the insert is lifted by the cutting forces. Further is it disclosed that the triangular basic shape of the hole gives a precise positioning of the insert. However, the shape of the hole is not meant to give a pre-positioning of the insert before it is fastened. Moreover, no possibility is given to deviate from the insert seat position predetermined by the triangular hole shape.
According to the Swedish patent application No. 9301400-9, the top opening of the insert hole is shaped with recesses in order to provide a well defined two point abutment between screw and insert. However, these recesses do not extend farther down in the insert hole, wherefore they in no way serve to enable a certain prepositioning of the insert before the screw is tightened.